1. Field of the Invention
The present invention relates to a pickup arm or tone arm for use in phonographs or the like and, more particularly, to a pickup arm provided with an inside-force cancelling device and a stylus-force application device, both devices having a new and novel mechanisms for generating and controlling magnetic forces.
2. Description of the Prior Art
In reproducing the recorded sound from a record by the use of a pickup arm, the tip of the stylus is pulled in the direction tangent to the groove by a force f proportional to the stylus force. If the angle between the line connecting the vertical pivot of the pickup arm with the stylus point and the tangent to the groove is assumed to be .phi., a component of force f'=f tan .phi. will act toward the center of the record. The component of force f' is called the inside force. The inside force causes the stylus to laterally press the side walls of the grooves thereby deteriorating the quality of reproduction of the recorded sound.
The angle .phi. varies with the diameter of the groove. Therefore, the inside force also varies as the stylus point moves from the outside portion of the record toward the center thereof or as the pickup arm advances. In other words, the inside force is at a maximum when the stylus is at the outside portion of the record and decreases as the stylus approaches the center of the record as shown in FIG. 9. In order to cancel or eliminate the inside force, various technical means have been proposed to give the pivot of the pickup arm a torque having a characteristic curve opposite to that of the inside force. However, these inside-force cancelling means have some disadvantages. For instance, those with balance weights have a disadvantage in that they lose balance when the pivot of the pickup arm is placed obliquely. In other words, when the plane on which the pickup arm rotates is not horizontal, those with springs have a disadvantage in that it is difficult to obtain a repulsion performance sufficient to cope with close control, while those of the magnetic repulsion type are higher in repulsion performance than those of the above-mentioned type but not satisfactory.
Meanwhile, conventional means for controlling the stylus force which causes the occurrence of the inside force are usually divided into two types, namely static balance type and dynamic balance type. The static balance type means balance the weight of the arm containing the pickup cartridge with that of a counterweight. The dynamic balance type means utilize the repulsion of springs. These means, however, also have disadvantages similar to those mentioned hereinabove.
As is well known, the record is generally made of synthetic resin and, strictly speaking, is not uniform in thickness and therefore the stylus moves up and down while varying the stylus force. The smaller the stylus force, the smaller the inside force is. However, if the stylus force is too small, the stylus point cannot perfectly follow the wavy movement of the record surface. Therefore, it is required to provide a pickup arm mechanism that can keep the stylus force proper within the allowable limit.
The stylus-force application device of the present invention is of the dynamic balance type. The basic mechanism and principle on which it depends are similar to those of the inside-force cancelling device mentioned above.
This stylus-force application device is much superior in performance to the conventional device because such contains no resonant element such as a spring mechanism as in the conventional device which deteriorates the reproduced sound and can produce the stylus force by contactless means.
With this device, the stylus force may be closely controlled in an increment of about 10 mg. Accordingly, this device can achieve the so-called small-stylus-force control required for the high-speed rotation of the video disk record and can remarkably increase the service lives of the stylus and the record.
Therefore, the present invention is intended to eliminate the above-mentioned disadvantages of the prior art.